A liquid crystal display mode which has meanwhile found widespread interest and commercial use is the so-called PS (“polymer sustained”) or PSA (“polymer sustained alignment”) mode, for which the term “polymer stabilised” is also occasionally used. In PSA displays an LC medium is used that contains an LC mixture (hereinafter also referred to as “host mixture”) and a small amount, typically <1% by weight, for example 0.2 to 0.4% by weight, of a polymerisable component comprising one or more polymerisable compounds, preferably polymerisable monomeric compounds, very preferably polymerisable mesogenic or liquid-crystalline compounds, also known as reactive mesogens or “RMs”. After filling the LC medium into the display, the polymerisable compounds are polymerised or crosslinked in situ, usually by UV photopolymerisation, preferably while a voltage is applied to the electrodes of the display. The polymerisation is carried out at a temperature where the LC medium exhibits a liquid crystal phase, usually at room temperature. As a result the polymerised or crosslinked RMs will phase-separate from the LC medium and form a layer on the inner surface of the substrates, where they induce a pretilt angle of the LC molecules relative to the substrates.
The PS(A) mode is meanwhile used in various conventional LC display types. Thus, for example, PS-VA (“vertically aligned”), PS-OCB (“optically compensated bend”), PS-IPS (“in-plane switching”), PS-FFS (“fringe-field switching”), PS-UB-FFS (“Ultra Brightness FFS) and PS-TN (“twisted nematic”) displays are known. The polymerisation of the RMs preferably takes place with an applied voltage in the case of PS-VA and PS-OCB displays, and with or without, preferably without, an applied voltage in the case of PS-IPS displays. In case of PS-OCB displays, for example, it is possible for the bend structure to be stabilised so that an offset voltage is unnecessary or can be reduced. In case of PS-VA displays, the pretilt has a positive effect on the response times. For PS-VA displays, a standard MVA (“multidomain VA”) or PVA (“patterned VA”) pixel and electrode layout can be used. It is also possible to use only one structured electrode without protrusions, which significantly simplifies production and improves contrast and transparency.
Furthermore, the so-called posi-VA mode (“positive VA”) has proven to be particularly suitable. Like in conventional VA and PS-VA displays, the initial orientation of the LC molecules in posi-VA displays is homeotropic, i.e. substantially perpendicular to the substrates, in the initial state when no voltage is applied. However, in contrast to conventional VA and PS-VA displays, in posi-VA displays LC media with positive dielectric anisotropy are used. Like in IPS and PS-IPS displays, the two electrodes in posi-VA displays are arranged only on one of the two substrates, and preferably exhibit intermeshed, comb-shaped (interdigital) structures. Upon application of a voltage to the interdigital electrodes, which create an electrical field that is substantially parallel to the layer of the LC medium, the LC molecules are switched to an orientation substantially parallel to the substrates. In posi-VA displays, a polymer stabilisation by addition of RMs to the LC medium, which are then polymerised in the display, has also proven to be advantageous. Thereby a significant reduction of the switching times can be achieved.
PS-VA displays are described for example in K. Hanaoka et al., SID 04 Digest 2004, 1203, 233-236, EP1170626 A2, U.S. Pat. Nos. 6,861,107, 7,169,449, US2004/0191428A1, US2006/0066793A1 and US2006/0103804A1. PS-OCB displays are described for example in T.-J-Chen et al., Jpn. J. Appl. Phys. 45, 2006, 2702-2704 and S. H. Kim, L.-C-Chien, Jpn. J. Appl. Phys. 43, 2004, 7643-7647. PS-IPS displays are described for example in U.S. Pat. No. 6,177,972 and Appl. Phys. Lett. 1999, 75(21), 3264. PS-TN displays are described for example in Optics Express 2004, 12(7), 1221.
PSA displays can be operated as either active-matrix (AM) or passive-matrix (PM) displays. In case of AM displays individual pixels are usually addressed by integrated, non-linear active elements like for example transistors (such as thin-film transistors or “TFTs”), whereas in PM displays individual pixels are usually addressed by the multiplex method as known from prior art.
A PSA display preferably comprises an alignment layer on one or both of the substrates forming the display cell. The alignment layer is usually applied on the electrodes (in case such electrodes are present) such that it is in contact with the LC medium and induces initial alignment of the LC molecules. The alignment layer may comprise or consist of, for example, a polyimide, which may also be rubbed or prepared by a photoalignment method. The alignment layer is usually formed by depositing a solution of an alignment layer material like for example polyimide, or a precursor thereof like for example a polyimide precursor, on the substrate, for example by coating or printing methods, and curing the alignment layer material or its precursor by exposure to heat and (or actinic radiation, for example UV radiation.
In particular for monitor and especially TV applications optimisation of the response times, but also of the contrast and luminance (and thus transmission) of the LC display is still desired. The PSA method can provide significant advantages here. Especially in case of PS-VA, PS-IPS, PS-FFS and PS-posi-VA displays, a shortening of the response times, which correlate with a measurable pretilt in test cells, can be achieved without significant adverse effects on other parameters.
Curved displays have gained interest for use in the TV market. Curved displays can enhance the optical viewing experience for viewers sitting off-center, because it is possible to keep a uniform distance between the viewer and the display. Thereby it is possible to reduce the image distortion which is caused in case of conventional, uncurved displays by the phenomenon that the sides of the display are farther away from the viewer than its center.
In curved displays the radius of the top and bottom substrates should be different from each other in order to maintain a constant cell gap. This is exemplarily and schematically illustrated in FIG. 1. As shown in FIG. 1(a) a flat panel display with two plane-parallel substrates (11, 12) usually has a constant cell gap (indicated by the double arrows) over the entire display area. On the other hand, as shown in FIG. 1(b) if in a curved display the curved substrates (11, 12) would have the same radius of curvature, the cell gap (indicated by the double arrows) would be smaller at the edges than at the center of the panel. Therefore, it is necessary in a curved display to use two substrates as shown in FIG. 1(c), where the radius of curvature of the bottom substrate (12) is longer than the radius of curvature of the top substrate (11), i.e. the degree of curvature of the bottom substrate (12) is lower than the degree of curvature of the top substrate (11).
In curved displays of the PSA mode, especially in curved displays of the PS-VA mode, the problem arises that different curvature of the two substrates will lead to a distortion of the pretilt angle between the top and the bottom substrate.
This is illustrated in FIG. 2, which exemplarily and schematically depicts a PSVA display with a top substrate (21) and a bottom substrate (22), and LC molecules (23) oriented with a certain pretilt angle relative to the top and bottom substrates (21, 22). As shown in FIG. 2(a), in a flat panel PS-VA display with plane-parallel substrates (21, 22) the pretilt angle α of the LC molecules (23), which is generated by the PS-VA process, is the same at the top substrate (21) and the bottom substrate (22). On the other hand, as shown in FIG. 2(b), in a curved display with a constant cell gap the LC molecules (23) at the top substrate (21) and the bottom substrate (22) are aligned at the same pretilt angle α relative to the tangent lines (24, 25) on the top and bottom substrate (21, 22) (indicated by the broken lines, hereinafter also referred to as “base lines”). However, the slopes of the base lines (24, 25) on the top and bottom substrate (21, 22) relative to the normal (26) at the center of the display (indicated by the broken line) differ from each other due to the fact that the top substrate (21) and bottom substrate (22) have different radius of curvature. Consequently, if as shown in FIG. 2c parallel base lines (24, 25) would be applied to both the top substrate (21) and the bottom substrate (22), the pretilt angle α′ at the top substrate (21) and the pretilt angle α″ at the bottom substrate (22) would be different from each other, with α′ being higher than α″, causing a distortion of the orientation of the LC molecules (23).
Therefore there arises a need to control or change the pretilt angle of the LC molecules relative to the top substrate and the bottom substrate in a PSA display, especially in a curved PSA display, such that an undesired distortion of the orientation of the LC molecules can be reduced or prevented.
The invention is based on the object of providing improved means and methods for manufacturing PSA displays and improved materials used therein, like RMs, LC host mixtures, additives, and LC media comprising the same, which can contribute to solve the above-mentioned problem.
Another object of the invention is to provide improved means and methods for manufacturing PSA displays and improved materials used therein which have high specific resistance values, high VHR values, high reliability, low threshold voltages, short response times, high birefringence, show good UV absorption especially at longer wavelengths, allow quick and complete polymerisation of the RMs contained therein, allow the generation of a low pretilt angle as quickly as possible, enable a high stability of the pretilt even after longer time and/or after UV exposure, reduce or prevent the occurrence of image sticking in the display, reduce or prevent the occurrence of ODF mura in the display, and reduce or prevent distortion of the orientation of the LC molecules.
The above objects have been achieved in accordance with the present invention by the methods and materials as described and claimed in the present application.
It has surprisingly been found that the above-mentioned problem can be solved by providing a method of manufacturing a PSA display, and a method of controlling the pretilt angle at the top and bottom substrate in a curved PSA display, as disclosed and claimed hereinafter.
In order to solve the above-mentioned problem of controlling the pretilt angles in curved displays as described above, the pretilt angle at the top substrate (hereinafter also referred to as “first substrate”) should be lower than the pretilt angle at the bottom substrate (hereinafter also referred to as “second substrate”).
In the method according to the present invention this is achieved by providing a PSA display comprising on each substrate an alignment layer that is formed from an alignment layer material or a precursor thereof. To the material forming the alignment layer is added either a polymerisation inhibitor which inhibits polymerisation of the polymerisable component of the LC medium, or a polymerisation initiator which initiates the polymerisation of the polymerisable component of the LC medium. For example, if an inhibitor is added only to the material that forms the alignment layer on the first substrate, polymerisation of the polymerisable component of the LC medium is inhibited in proximity to the first substrate, and a lower pretilt angle is generated at the first substrate. On the other hand, if an initiator is added only to the material that forms the alignment layer on the second substrate, polymerisation of the polymerisable component of the LC medium is inhibited in proximity to the first substrate and a higher pretilt angle is generated at the second substrate. Both methods can also be combined.
The method according to the present invention can also be used in flat panel displays without curvature of the substrates, in case it is necessary to control or change the pretilt angle at the substrates.
In addition, the methods and materials as disclosed and claimed hereinafter yield PSA displays with high specific resistance values, high VHR values, high reliability, low threshold voltages, short response times, high birefringence, good UV absorption especially at long UV wavelengths, quick and complete polymerisation of the RMs, quick and strong pretilt angle generation, high pretilt angle stability over long time especially under stress caused by heat, voltage and/or UV light, reduced image sticking, reduced ODF mura, and reduced distortion of the orientation of the LC molecule.
Moreover, the methods and materials according to the present invention facilitate a quick and complete UV-photopolymerisation reaction in particular at low UV energy and/or longer UV wavelengths in the range from 300-380 nm and especially above 340 nm, which are considerable advantages for the display manufacturing process.